321 



THE ORIGIN OF THE BRITISH TRIAS.* 



Professor T. G. BONNEY, ScD., LL.D,, F.R.S. 



The three sub-divisions of the Bunter, whether east or west of 

 the Pennine Rang-e, apparently unite to the south of it, and thin 

 out as they approach the southern parts of Warwickshire, 

 Staffordshire, and Leicestershire. Their equivalents are fairl}^ 

 well developed in Devonshire, but apparently thin out in a 

 similar wedg-e-like manner towards the north and north-east, 

 not reaching" the Bristol Channel. The upper and lower mem- 

 bers in the northern area are sandstones, g^enerally red, often 

 conspicuousl}' current-bedded, but without pebbles, the grains 

 being- frequently wind- worn. The pebble-bed between them 

 reaches a thickness of looo feet near Liverpool — where, how- 

 ever, sand dominates over pebbles — is about 300 feet thick in 

 Central Staffordshire, and rather overlaps the Lower Bunter 

 sand. The writer describes the lithological characters of the 

 pebbles, and discusses the reasons for and against deriving 

 them either from a southern or south-western soiu'ce, like those 

 in the Devon area, or from any region, either exposed or buried, 

 in their more immediate neighbourhood, maintaining a northern 

 origin to be more probable. The Keuper group, both sand- 

 stones and marls, extends without interruption (except for the 

 sea) from Devonshire to Yorkshire on the one hand, and to 

 Antrim on the other. 



The author considers the Bunter to be fluviatile rather than 

 lacustrine deposits, chiefly formed b}' large rivers. Two of 

 these flowed from a mountain region, of which Scotland and the 

 extreme north of Ireland are fragments, and a third from a 

 similar region to the south-west of Britain. Deposits com- 

 parable with the Bunter, and especially the pebble-bed, may be 

 found on the border of the Alps, and those rivers probably 

 traversed (at any rate early and late in the Bunter epoch) arid 

 lowlands, from which, if not absorbed, they ma}' have escaped 

 by some channel now buried under south-eastern England. 

 The Keuper sandstones, as he shows, indicate the setting in of 

 inland sea-conditions, the Red Marls being generally regarded 

 as deposited in a great salt lake. These, like the clays of the 

 Jurassic system were probably derived from the mountain 

 ranges which had previously supplied sand and pebbles. 

 In fact, the physical and climatal conditions of the Trias — 



* Abstract of a paper read to Section C of the Meeting- of the British 

 Association, York. 



1906 September i. 



